Test and measurement devices, such as oscilloscopes and spectrum analyzers, for instance, accept input signals and display them or other data or signals derived from them on a display for viewing and analysis by a user.
For instance, signals such as Radio Frequency (RF) signals may be modulated with a digital signal to create a modulated RF signal capable of carrying data from a source to a destination. One method of deriving the original digital signal from the modulated RF signal is by demodulating the RF signal. If desired, the original data used to make the digital signal may also be reconstructed from the demodulated digital signal.
Two widely used schemes for encoding the digital signal into an RF signal are phase modulation (PM) and frequency modulation (FM), using widely known methods such as phase shift keying (PSK) and frequency shift keying (FSK), respectively. In those examples, to make an RF signal that carries the digital signal, the digital signal (or a smooth-filtered version of the digital signal) is modulated using PSK or FSK along with a carrier wave into the resultant modulated RF signal. Later, the receiving device de-modulates the received RF signal and extracts the previously encoded digital signal. In a test and measurement device, the decoded digital signal is typically shown on the display. Additionally, data from the digital signal may be generated and stored. Viewing the modulating digital signal waveform and subsequent effects of the circuitry and channel through which the data passed is very helpful, especially when testing and measuring data communication equipment.
A problem exists, however, when the test and measurement device attempts to decode an RF signal that is not carrying data. Oftentimes data is placed on an RF signal in bursts, where data is present during data bursts and not present at other times. When the test and measurement device attempts to display the demodulated digital signal but no data is present, the device shows such a condition as noise. Phase detection of noise, when no data is present, produces a random phase between +/−180°, and frequency demodulation of noise yields random frequencies. This displayed noise makes it difficult for users of test and measurement devices to effectively extract useful information from the display screen.
Embodiments of the invention address these and other limitations of the prior art.